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Abstract Long‐term watershed experiments provide the opportunity to understand forest hydrology responses to past logging, road construction, forest regrowth, and their interactions with climate and geomorphic processes such as road‐related landslides. We examined a 50‐year record from paired‐watershed experiments in the H. J. Andrews Experimental Forest, Oregon, USA in which 125 to 450‐year‐old conifer forests were harvested in the 1960s and 1970s and converted to planted conifer forests. We evaluated how quickflow and delayed flow for 1222 events in treated and reference watersheds changed by season after clearcutting and road construction, including 50 years of growth of planted forest, major floods, and multi‐decade reductions in snowpack. Quickflow runoff early in the water year (fall) increased by up to +99% in the first decade, declining to below pre‐harvest levels (−1% to −15%) by the third to fifth decade after clearcutting. Fall delayed flow responded more dramatically than quickflow and fell below pre‐treatment levels in all watersheds by the fifth decade, consistent with increased transpiration in the planted forests. Quickflow increased less (+12% to 70%) during the winter and spring but remained higher than pre‐treatment levels throughout the fourth or fifth decade, potentially impacted by post‐harvest burning, roads, and landslides. Quickflow remained high throughout the 50‐year period of study, and much higher than delayed flow in the last two decades in a watershed in which road‐related changes in flow routing and debris flows after the flood of record increased network connectivity. A long‐term decline in regional snowpack was not clearly associated with responses of treated vs. reference watersheds. Hydrologic processes altered by harvest of old‐growth conifer forest more than 50 years ago (transpiration, interception, snowmelt, and flow routing) continued to modify streamflow, with no clear evidence of hydrologic recovery. These findings underscore the importance of continued long‐term watershed experiments.
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Comparing the Dynamics of Dissolved Organic Carbon in Harvested and Unharvested Watersheds within the Oregon Cascades
Clearcutting and other land-use changes are known to release terrestrial carbon and mobilize organic carbon into streamwater, significantly augmenting aquatic carbon levels in the short-term. However, little is known about the lasting impacts of forest management decisions on the riverine concentration levels of Dissolved Organic Carbon (DOC). Here we compare data from HJ Andrews Experimental Forest, a long-term ecological research (LTER) site located in the Oregon Cascades. We paired stream chemistry and discharge measurements spanning 15-30 years. Two watersheds that were 100\% clear-cut 40-50 years ago (WS01 and WS10) were compared with their unharvested and controlled counterparts (WS02 and WS09). Temporal analysis showed that, on average, DOC concentrations in the old-growth watersheds are notably higher than their harvested analogs to this day. This suggests even though clearcutting can release DOC from soil and vegetation to water, the terrestrial organic carbon stock is ultimately depleted post-clearcutting resulting in lower DOC concentrations. Concentration-discharge (CQ) analysis also revealed a sharp difference in behaviors between watersheds 1 and 2, with WS01 exhibiting a slight flushing pattern bordering on hysteresis while WS02 displayed a pronounced dilution pattern. Based on the shallow-deep hypothesis (Zhi et al. 2019; Zhi and Li, 2020) this indicates that the old-growth watershed has a pronounced groundwater DOC source, and clearcutting could have altered this source within WS01 and significantly lowered baseflow organic carbon concentrations. However, it should be noted that WS09 and WS10 displayed DOC behavior similar to that of WS01, which could also signify that the previously mentioned opposing CQ behaviors are a result of some underlying geological or lithological contribution unique to WS02. These competing hypotheses will be further tested using a watershed scale reactive transport model HBV-BioRT.",
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- Award ID(s):
- 2121621
- PAR ID:
- 10345937
- Date Published:
- Journal Name:
- American Geophysical Union Annual Conference
- Volume:
- 2021
- Issue:
- H45C--1198
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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